Optical test units that sense light scatter resulting from a fluid test sample (e.g., an aerosol test sample) being illuminated by light generated by a light source (e.g., laser) are known. Known optical test units include photometers, biological detectors, particle counters and dust monitors. However, known optical test units have a number of limitations and/or disadvantages. Preferred forms of the present invention have been developed to significantly reduce the limitations and/or overcome one or more disadvantages of known optical test units.